One of the techniques known in the conventional art is a semiconductor light source module capable of ensuring that the laser beam emitted from a semiconductor laser as a light source is converged through a light converging optical system on the end of an optical waveguide forming an optical transmission line therein, such as an optical fiber and an SHG element.
As shown in FIG. 1, an example in the conventional art is given by the light source wherein light 2 from a laser diode 1 is converged on entrance end 5a of the optical waveguide of a second harmonic generator (SHG) element 5 through the first lens 3 and second lens 4 to be propagated in the optical waveguide, thereby the second harmonic light is outputted from the other end of the optical waveguide.
In the light source module in which an optical waveguide propagates light from the light source to output the light, the light from the light source is requested to be converged onto the entrance end of the optical waveguide and the light flux is requested to be effectively coupled with the optical waveguide.
However, in the assembling of passive alignment, a light-converged spot on the entrance end 5a is displaced due to an assembling error. Therefore, it reduces the coupling efficiency, or prevents light from coupling with the optical waveguide at all. For example, the single mode fiber has a core whose diameter or mode field diameter does not exceed 10 μm and the multi-mode fiber has a core whose diameter or mode field diameter is about 50 μm. Further, some optical waveguides in a device such as SHG element have a core whose diameter or mode field diameter is several μm. Since light is required to enter this microscopic area, an optical element such as a semiconductor laser, lens and waveguide is also requested to be adjusted on the microscopic level. Thus, an excellent coupling performance is hardly ensured in passive alignment, and the yield rate is also poor, which are problems.
To solve these problems, attempts have been to mount a lens moving mechanism on the lenses 3 and 4 and to adjust the positions of the lenses 3 and 4 so that the light-converged spot will enter the entrance end 5a, whereby the coupling efficiency is optimized.
Unexamined Japanese Patent Application Publication (JP-A) No. 2005-222049 discloses an invention wherein a weak lens is arranged between the first lens 3 and second lens 4, and this weak lens is shifted in the direction of optical axis. In this invention, the amount of change in coupling efficiency is small with respect to the shift of the weak lens, hence adjustment precision is alleviated and adjustment is facilitated.
Further, JP-A No. 2003-338795 discloses a technique as follows: when the laser beam emitted from a semiconductor laser is converged onto the end of an optical fiber through a light-converging optical system, light reflected from the end or light passing through the optical transmission line is detected and the light-converging optical system is driven in the direction perpendicular to the optical axis, whereby a spot is properly converged onto the end of the optical fiber.
The technique in the invention described in JP-A No. 2005-222049, requires one weak lens additionally to alleviate the adjustment precision. This signifies an increased number of lenses, more complicated structure, upsized configuration and increased cost. The technique in the invention disclosed in JP-A 2003-338795, in which the lens is moved by an actuator, can cause a deterioration in coupling due to an aberration caused when the lens is driven and adjusted. This may increase sensitivity in the lens drive and adjustment. To solve the problem, two or more lenses can be used to form the optical system for converging the light flux from the light source onto the end of the optical fiber or the waveguide of the SHG element. When a plurality of lenses are moved, ideal adjustment of the light flux converging position can be achieved if each lens can be moved in a three-dimensional space. The drive apparatus for moving each lens in a three-dimensional space involves the problem of complicated structure and upsized configuration. Thus, there has been a demand for achieving the simplest possible structure of the drive apparatus by restricting the lens drive direction, for example.
The following discusses the relationship between a direction of spot displacement and coupling efficiency.
FIG. 2 is a chart showing a change in the coupling efficiency with respect to spot displacement in the direction of optical axis (Z-axis direction in FIG. 1) and in the direction perpendicular to the optical axis (X- and Y-axis directions in FIG. 1) when the mode radius of the light-converged spot and that of the SHG waveguide each are 2.5 μm in the optical system of FIG. 1. As is apparent from this chart, a change in coupling efficiency with respect to the displacement in the direction of optical axis is more gradual than that with respect to the displacement in the direction perpendicular to the optical axis.
Based on this finding, the module can be simplified if the direction of correcting the lens position is restricted to the direction perpendicular to the optical axis and the mechanism of shifting the lens in the direction of optical axis is not used. Adjustment precision in the Z-axis direction is less severe. Accordingly, if the lens position is adjusted in the X- and Y-axis directions along after high-precision assembling, it is possible to produce a module of high coupling efficiency with the optical waveguide.
Despite the highest possible assembling precision in the Z-axis direction, when the lens position is adjusted in the direction perpendicular to optical axis, the light-converged spot is displaced in the direction perpendicular to optical axis as well as in the direction of optical axis. This results in poor coupling efficiency.
To solve this problem, in the structure wherein, for lens position adjustment, the lens is moved in the direction perpendicular to optical axis alone, not in the direction of optical axis, it is required to design a lens capable of reducing the spot displacement in the direction of optical axis resulting from the lens movement in the direction perpendicular to optical axis.
Similarly, in the structure wherein the lens is moved by an actuator, as in the invention of JP-A No. 2003-338795, it is considered that the optical system for converging the light flux from the light source onto the end of the optical fiber or the waveguide of the SHG element is made up of two lenses—a lens for collimating the light source and a lens for coupling the collimated light to the waveguide, for example. These two lenses can be driven for correction by the actuator in the X direction perpendicular to the optical axis, and in the Y-direction perpendicular to the optical axis and X axis. This will be discussed in the following. Such a drive system will correct the spot displacement in the X direction and Y direction.
If the lens is displaced in the X direction or Y direction, focal position is shifted in the direction of optical axis due to the curvature on the image surface. However, when the lens is not driven in the direction of optical axis for the sake of simplification of the drive apparatus, the focal position is hardly adjusted by the curvature of image surface. Accordingly, it hardly control increase of the spot diameter and a part of the light will come off the edge of the fine waveguide of the SHG element or the end of the optical fiber, with the result that the coupling efficiency is reduced.
To put it more specifically, consider the case wherein correction is made by shifting the first lens on the semiconductor laser side in the X direction and the second lens of the SHG element in the Y direction. If the second lens alone is shift in the Y direction, misalignment in the Y direction occurs to the semiconductor laser, first lens and SHG element. In this case, even if the second lens is shifted in the Y direction for correction, the decentering error of the second lens in the Y direction remains uncorrected. In the optical system without the decentering error being corrected, the coupling efficiency will be deteriorated by the generation of aberration and the production yield rate will be reduced.